Problem: 5 notebooks cost $6.55. Which equation would help determine the cost of 4 notebooks?
There are several equations that could help determine the cost, each with a slightly different approach. We know the cost of 5 notebooks. We want to know the cost of 4 notebooks. We can write the numbers of notebooks as a proportion: $\dfrac{5}{4}$ We know 5 notebooks costs $6.55. We can let $x$ represent the unknown cost of 4 notebooks. The proportion of these costs can be expressed as: $\dfrac{\$6.55}{x}$ The cost changes along with the number of notebooks purchased, and so the two proportions are equivalent. $\dfrac{5}{4} = \dfrac{\$6.55}{x}$